Report of the Board of Shell Fish Commissioners 113 



means of a three-arm protractor, 35 or position finder, the posi- 

 tions of the launch at the end of two or three minute intervals, 

 while running the sounding lines, are plotted on this boat sheet 

 by the chief engineer, the locations of the launch being ascer- 

 tained by means of sets of angles taken simultaneously by the 

 engineers observing three signals on shore with sextants. 36 By 

 connecting consecutive positions, as they are plotted, with 

 straight lines the course of the lines of soundings is shown on 

 the boat sheet. 37 



The three-point problem, which is involved in locating each 

 of the positions occupied 'during the survey, is illustrated by 

 figure 7, in which X represents the launch and A, B and C 

 the signals on shore. The angles AXB and BXC are those 

 taken by the engineers with sextants to locate their position at 

 X. The right-hand angle BXC (75 48') is transferred to the 

 vernier between the middle arm and right arms of the pro- 

 tractor and the left-hand angle AXB (58 16') is transferred 

 to the vernier between the middle and left arms. When the 

 protractor is thus set and laid on the chart in such position 

 that the left arm passes through the plotted position of signal 

 A, the middle arm through the plotted position of signal 5 and 

 the left arm through the plotted position of signal C the point 

 in which the three arms of the protractor meet, the centre of 

 the hub, is the point on the chart representing the position of 

 the boat when the sextant angles were taken. 



All angles taken during the survey by the engineers* are 

 transferred to the protractor and plotted on the boat sheets by 

 the chief engineer, while the assistant engineer records the 

 angles in an angle record book. 



ssA three-arm protractor (figure 5) is an instrument used for plot- 

 ting observations with sextants of two angles to three known points 

 for the location of the point of the observer. 



The description of the theory of the sextant and protractor and their 

 use in hydrography requires the use of language too technical to be of 

 general interest. 



36A sextant (figure 4) is an instrument constructed for measuring 

 the angle between two objects on shore (signals) from the position of 

 the observer. 



s'See also pages 23-25, Coast and Geodetic Survey Report, Survey of 

 Oyster Bars, Anne Arundel County. 



